Generation of microwaves



April 1953 J. R. PIERCE 2,635,206

GENERATION OF MICROWAVES Filed Jan. 6, 1949 FIG.

33 FOCUS/NC COIL INVENTOR J. R. PIERCE v6 ATTORNEY DECREASl/VG ,u

Patented Apr. 14, 1953 UNITED STATS ATENT GFFIQE Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application January 6, 1949, Serial No. 69,481

14 Claims. 1

This invention relates to the production of ultra-high-frequency oscillations through the use of interacting electron streams in an evacuated enclosure.

An object of the invention is to generate electromagnetic waves efiiciently at wavelengths as short as a centimeter or less.

The invention makes use of the discovery which was disclosed in the application of W. B. Heben streit and J. R. Pierce, Serial No. 38,928, filed July 15, 1948. In accordance with that discovery, a space charge wave can be propagated along an electron stream, with an amplitude that increases with distance of travel, when a second electron stream of a different velocity is made to interact with the first stream, provided certain quantities involved are properly proportioned. In particular, a sufficiently high electron current density should be used and the difference between the velocities of the two streams should be sufliciently great.

In accordance with the present invention, oscillations are generated by methods and means utilizing the property of amplification which is possessed by devices of the type disclosed in the above-noted I-Iebenstreit-Pierce application. In devices which embody the present invention, two interacting electron streams of different velocities serve to amplify, along a path of travel, a disturbance which appears in either or both of them and at least a portion of the amplified disturbance is conveyed back in electron stream form to the beginning of the path to reinforce the initial disturbance for the purpose of creating sustained oscillations.

Although the actual mechanism of energy interchange between electron streams appears to be highly complex and involved, embodiments of the present invention which are to be described may be considered, for purposes of explanation, to operate in the following manner. Two streams of electrons are projected along a predetermined path of travel at different velocities in a coupled relationship. If a disturbance of any kind appears in one of the streams, interaction of electrons will produce a corresponding disturbance in the other stream. When the disturbance appears on the second stream, electron interaction will reinforce the disturbance on the first stream, causing it to increase in amplitude. When the increased disturbance appears on the first stream, electron interaction will then reinforce the disturbance on the second stream, and so on. As a result, the disturbances on both streams increase in amplitude as the streams progress along the path of travel. If initial disturbances appear in both streams, a similar phenomenon occurs. When the initial disturbance appearing in one or both of the streams amounts to a velocity modulation, a recurrent bunching of electrons or density modulation takes place, with the bunching becoming more pronounced as the streams progress along the path. This phenomenon may be referred to as a space charge wave of negative attenuation, or, in other words, a space charge wave with an amplitude that increases with distance of travel.

The increased disturbances on the electron streams at the end of their path of travel amount to an amplification of the initial disturbance.

When the amplified disturbances, or disturbances corresponding to the amplified disturbances, are

, disturbances involved increases with each passage along the path until limiting conditions are finally reached. At that point, due generally to non-linearities of the system, amplification resulting from energy interchanges between electrons of the two streams falls off and sustained oscillations result. Amplification, at that amplitude level, is just sufficient to counteract losses and to replace any power drained from the system to supply an external load. Unless controlled by a resonator or some similar device coupled to at least one of the electron streams, the oscillations will tend to occur at the frequency where the amplification over the path is greatest.

Initial disturbances which appear in the electron streams may be caused by spontaneous fluctuations in electron current density as the electrons leave an emitter surface, that is, shot noise. Disturbances from various other causes may also serve to initiate oscillations. Systematic disturbances such as, for example, velocity or density modulation may be produced by a resonator coupled to at least one of the streams after oscillations have begun to build up and serve to add a great degree of stability to the system. In such an event, the oscillation frequency will generally be governed by the frequency to which the rest:- nator or other such device is tuned.

Although specific mention has thus far been i ticles traveling in coupled relationship at different average velocities will serve to support the previously-mentioned space charge wave of negative attenuation. Thus, amplification and oscillation may be obtained even if, for example, one of the above-noted electron streams is replaced by a stream of slowly moving positive ions.

It should also benoted that, dueto spacechargev efiects, the; two, electron. streams employed, in many embodiments of the present invention do not have discretely different velocities, but rather,

each stream generally includes a range of velocities. if suflicient electrons of each stream tendlto concentrate at velocities intermediate inltheirrespective velocity ranges, thus producing, an over-all,

multiple-peaked rather than single-peaked velocity distribution curve. the electron velocities of one stream, or a velocity at which electrons of that stream tend to concentrate, may, in some, instances, approach zero.

In' general; .the twointeractingstreams xofelectrons or' charged particleswillbe: either, intermingled or in closeproximity with one another:

Itihas been found" that; as a general rule,- in the absence'of other coupling-mechanism the amplification or'gain that takes-placeas the-twostreams" progress along'theirpathoftraveldecreases rapidly as the separation distance between'the two streams increases:

The nature and objects offthepresent inventionwillappear more fully fromithe following detailed description of two specificembodiments: In'thedrawings:

Fi ljsa longitudinal fcross=sectionfview "ofan' embodiment of the invention in which two, initiallyoppositelydirectedielectron streams are em' p ye Fig.4 2. is aplan sectionalview, taken on line 3 2-2jo f;Eig,-, 3; of an: embodiment of theinvention;

in :which the electron. pathis curved. and'jclosed backiuppn itself; and.

Fl'g, 3'is a cross-section view ofjth'e' structure showninFig; 2; taken on the'line 3 -35 Referringtoflig; 1, the tubeenvelope I flis'made of jmetal; such as copper, and is provided; with a relativelynarrow central bore II and with larger" diameter :endspaces I 2 and I 31 Located j in the end? space I2. isra ,metalj I4" having an activated" face I5,;facing1towardicentral?bore. II; which serves;asa cathode when it isheated. A heating coilfIBfis ,provided'within cylinderj I4" and is. supplied with heating'current'frombattery ITby way of JeadsIB andfIS: liead'laiisfbrought outflfrom one terminalofthe coilT'I 6" through an'dconcentric'rwithja short metal tube 20 which is provided at its' extremity with a glass vacuumseal. The opposite terminal of: the coil I6 is joined'to the cathode cylinder It, from which lead? I 91's brought outfthrough ashort'length of' tubing and glass sealin thesame manner as lead" I 8'.

In-theoppositeend space I3; a second metal cylinder '21- having an activated iac'e 22; facing central boreI I'; which servesas a second cathode is provided? A heating1coil"2,35is located within cylinder zl A lea'd Mfrom the heating coil23' is broughtoutthrough a glass bead in the end of "aishorttube joined to the envelope .IIJ'and is connectedfto one pole ofai battery 2'5." The other end'jiof coil 23"is connected. to vcathode cylinder 2|, whichfiis connectedfthrough a lead 26 to the other-pole of battery zfrand to an intermediate.

I: The most positiveterminalofbattery, 2'Iisl.

Those velocity ranges may b'etcontiguous.

It is contemplated: thatconnected to envelope Ill. Cathode 22 is heated partly by power from battery 25 and partly by bombardment by electrons from cathode I5.

An annular resonant cavity 28 is provided in the envelope I0 along the path defined by central bore II, preferably nearer the more positive cathode 22, and opens into the space II through a narrow,circumferentialslot-29a This resonant cavity communicates with the; interior 'of awave guide 30 through an orifice 3I which may be provided with a cover glass 32 suitably sealed to the tube It to enable the interior of the tube ID to be evacuated. The end of wave guide 30 is so shapedas to provide asnug fit against envelope Iogandian airrtightseal'is provided. A magnetic focusing, coil 33' is.,applied around the exterior of the tube It and is supplied with direct current from any-suitable source, not shown, for producing a longitudinal magnetic field directed along the axis of the tube It] for. focusing the electrons into a well defined cylindrical beam. A jacket may be provided around the'tube l nfor circulat ing'coolingfiuid:

In the operation -of""the deviceof Fig;- l' elecitronsemitted from the cathode I5 are acceleratedlengthwise of the tube due to the high voltageitm pressedupon" the tube-from"battery 2land'the number of these, which strike the inner walls" of the-tube in; the channel I I is" minimized due'to' thefocusingaction ofthe-magnetic 001L332 These electrons strike the second' cathodei 22.. Cat/hode"v 22-, due-to heating-causedby the 'impact "of elem-- trons "from cathode I5, to" heating fromicoi'l" 23:1, and to secondary emission; is' causedtdemit'releo trons; at a rate Which varies-appreciablyin" con-= formity with the'incomingstream: The emitted; electrons are projected toward the" left-"in the figure" alongthe channel j I I thatis; toward the first cathode I 5-; due to: the 'difierence in" direct current potential ,of the elements-22"'and I 0: It will be noted that the-beam fromcathode' I5 which travels toward the right is accelerated'by' the voltage ofthe full battery '2! 'while-thebeam: fronr cathode 22travelingztowardthe' left' is"ac:-;- celerated by a: fractional part ofthe "potentiai "of thefull' batters "27. Sinceth'epotential between" element z 27 and envelope'I fifis lessthanithat be= tween element I 5? and envelope I 0;: theelectrons? traveling toward -the left" will notz'each' the*cath'- ode I51 after-traversing central bOlB'II I", wherein". the "potential gradientisessentiallyzero; but will be turned'backbythe relatively 'steep'potentialf gradient existingrbetweemtheend of central more I I and cathode I5 and will travel towardthetrighiri along -with-but'at-a somewhat'lower velocityfth'an" the electrons originating from athodejfii The electrons of thebeam" emanating: from" cathode IS-interact with thoseofthe beamiemate nating from cathode 22 after th'eilatter electrons have reversed direction 'j ust before reaching cathsode I55 smallidis'turbances such as, formexample variations in electron, currentlden'sity causedfby cathode emission'irregul'arities and transienteffects introduced by. closing a rswitchwonnectingg, main, batery. 271 into, the. circuit tend to appear in b'othbeams and increase, in ,magnitude; as. the1 electrons comprising.,tlie beamsrprogresslto the 'ward the left is caused,jbylsecondaryemission and 1 by the varying spacercharge of..the beams, ape proaching cathode 22,.to.vary in.accordance with the amplifiedldis'turbancesr, Theaamplifiedvdisrturbances, are. thenconveyed backrb'y, the beam aeeaaoa virtually unaltered, to the other end of the path defined by central bore I I. By interaction of electrons, similar increased disturbances are caused to appear in the stream being emitted by cathode l after the stream emanating from cathode 22 reverses direction and starts back toward cathode 22. Further amplification then takes place due to interaction between the electrons of the respective streams as they travel once more toward cathode 22.

As the disturbance-bearing electron streams pass slot 29, they yield energy to resonator 28, tending to set up an alternating field therein. Once an alternating field is established in resonator 28, density modulations large in comparison with the initial disturbances appearing in the beams are impressed upon the beams as they pass slot 29. The frequency of operation is thus stabilized at the tuned frequency of resonator 28 by the time limiting conditions are reached and sustained oscillations occur. The generated oscillations may be extracted through wave guide 30 for application to a desired load circuit.

It should be noted that if the resonator 28 has a low Q, that is, the ratio of energy stored to energy lost per oscillation cycle is low, or is so coupled to wave guide 39 as to give bandpass or aperiodic characteristics, it will not control the frequency of oscillation. The oscillation mechanism will then be chiefly electromechanical and the frequency will be controlled by the beam diameter, beam current and voltage, and the velocity separation between the two beams, that is, the voltage between cathodes I5 and 22. Under these circumstances the frequency of oscillation can be changed by varying the beam current or voltage.

Calculations indicate that for wavelengths of the order of from five to ten centimeters, the channel might typically be ten inches long and one-tenth of an inch in diameter and might supply currents of from one to ten milliamperes. If the diameter is unduly large, the potential of the electron stream will be lower than that of the wall because of space charge. More important, if wa/v is too large (w=radian frequency, a=radius of hole in resonator, v=mean electron velocity), there will be too little coupling between electron streams and resonator. Battery 21 might have a voltage of about two hundred volts and cathode 22 could be about ten volts positive with respect to cathode l5.

Referring to Figs. 2 and 3, a doughnut-shaped or toroidal glass envelope 35 defines a closed circular path and is provided with cathodes 33 and 31, which are situated adjacent to one another at a point in the path defined by envelope 35. Cathodes 33 and 31 may be either indirectly heated or filamentary cathodes, the latter type being shown, which are supplied with heating current from a portion of the cells of battery 38. For this purpose the two filamentary cathodes 3B and 31 may be connected in series with a resistance 39 between them which is considerably higher than the resistance of either filament. When this is done the average voltage of the cathode 36 will be negative with respect to the average voltage of cathode 31 by substantially the voltage of the heating current source. Electrons from the two cathodes 36 and 31 are accelerated counter-clockwise in Fig. 2 by a grid member 40 which is displaced somewhat from cathodes 36 and 31 along the path of travel. Grid 40 is at positive potential with respect to both cathodes due to its connection to the positive pole 6 of battery 38. The electron stream starting from cathode 36 will be the high velocity stream and that from cathode 31 will be the low velocity stream because of the lesser difference in potential available for driving the latter with respect to the former.

The two electron beams indicated in Fig. 2 at 4| and 42 are constrained to move in a circular path by the use of a magnetic field perpendicular to the plane of the paper. This field is produced by windings 43 and 44, as shown in Fig. 3, applied respectively to the annular core members 45 and 46. In order to secure more accurate focusing of the beams, the magnetic field may be made weaker toward the outside of the path of travel than toward the inside by use of variable permeability core material having decreasing permeability toward the outer circumference as indicated in Fig. 3. This decrease in permeability may be accomplished in practice by making the core members of finely divided material or laminations and decreasing the ratio of magnetic material to filling as the outer circumference is approached. If the core members are made in the form of clock spring ribbon, greater separation between turns may be allowed at the outer circumference of the core members than further along toward the inner circumference. Since the magnetic field is stronger toward the inside of the path of travel than toward the outside, the inner stream 4| will move in a circle of smaller radius than will the outer stream 42. Coupling between the two streams is thereby better maintained over the whole path of electron travel.

For the purpose of both modulating the streams and taking off power, a resonant cavity 41 in annular form surrounds the tube 35 at a suitable distance beyond the filaments 36 and 31. The inner portion of the cavity is of restricted width and extends through the tube 35, its opposite walls comprising two grids 48 and 49. The generated oscillations are taken off through a coaxial line, the inner conductor 53 of which is shown coupled to the interior of the cavity 41.

In the operation of this device, small irregularities appear in the electron streams originating at cathodes 36 and 31 and gain in amplitude as the streams progress around the tube in interacting relationship. In the device illustrated, the path is continuous and closed, enabling the amplified disturbances to be conveyed by streams 4| and 42 themselves back to the beginning of the path. The amplified disturbances are further amplified in subsequent trips around the path and yield energy to cavity 41 as they cross gap 48-49 until, as with the Fig. 1 device, an alternating field is established in cavity 41 and the velocity modulations which the streams then receive as they cross gap 4849 are substantially greater than the original disturbances and control the frequency of oscillation. When limiting conditions are reached, gain along the path falls off, and stable sustained oscillations occur at the resonant frequency of cavity 41. Oscillations produced in cavity 41 are led off to a suitable load circuit over conductor 50.

Electrons are prevented from traveling in a reverse direction around the tube by the action of the magnetic field, which is in such direction as to throw the electrons starting to move in a. clockwise direction from cathodes 36 and 31 toward the outer wall of the tube 35.

The embodiments that have been disclosed are to be considered as illustrative and by Way of example rather than as limiting, inasmuch as avarietyor embodiments are. possible withinthe spirit and scopeof the invention.

What-is claimed is:-

1 A space discharge device comprising.- an enclosure defining a path of travel. for charged. particles, means within said enclosure for pro.- jecting: a pair of streams of charged particles along said path atrespectiize-diiferent velocities, one portion of. said path being coupled? regeneras tively-toan anterior portionof' said patlr by atv least one stream or charged particles, meanssituated along said path for systematically varying the density of th charged particle-flow of atl'east one of said streams, and means situatedv along: sai'd path for extracting electromagnetic energy -.frcmatleast one of said streams.

2'. Anoscillator comprising an evacuated toroidal -enclosure-defininga continuous closed path of travel forelectrons, a pair of electron-emis-.

sive elements within said enclosure, and means coupled to said elements for projecting a pair ornelect-ron streams from. said elements around said: path. continuously in the. same direction at respective different velocities.

3. An oscillator in accordance. with claim 2' which includes means situated along said. path for-systematicallyvarying the density or". the elec-.

tron flow. in at least. one of: said streams and means situatedialong said. path for withdrawing oscillatory electromagnetic. energy from. at. least oneoi-sailrstreams.

Anoscillator in. accordance. with. claim .2

in which. one ofi' said electron. emissive elements.

to; said. third electrode to hold it. at a. direct-.

current potentialpositive; with respect to those f: said-first and. second electrodes. and means adjacent to the; Path between: said. first and.

second electrodes to withdraw oscillatory elect-rome netic; ener y from at. least one. of..t;he electron.

tre ms which. m nate. from. said. first and $3.0? nd, electrodes; when energized.

6;. A. high, frequency oscillator in. accordance with claim 5. in; which said last-mentionedmeans comprises a ca-vityresonaton v LA microwave oscillator which; comprises a pair of. electron-.emissive electrodes spaced apart to. d fine. a path of electron; trav l, between them.

means; coupled to said electron-emissive; electrodes tohold them at different. direct-currentpotentials, a tubular electrode surrounding and extending; for substantially the whole length oi the.- path' between said; electronremrissiy elece trodes, means coupled to said tubular electrode to. hold it at a direct-current potential positive with respect to said electroneemissive electrodes, and an, electromagneticresonator disposed in coupled relationship with. at. least one of the electron streams which emanate from: said electron-emissive electrodes when energized.

8;. An. oscillation generator which comprises'an evacuated enclosure defining a path of travel for electrons,.;electrode .means within said enclosure 5.

to 'profect a pair of 'steams of electrons in the .claim 8; in which. said means to stabilize the oscillation frequency comprises -a,.-cavity resonaton l0; An: oscillation generator in. accordancewith claim 8m which. said means tostabilize thBJOSCilr;

lationfrequency and said energyabstraction; means comprise the; same cavity resonator;v

11., An oscillation generator which comprises; an'evacuated enclosure. defining a path. of travel for elec ron first; and second thermionicv oath.- odes situated at opposite ends of said path,.mea ns including a source; of direct. potential to main.- tain said second cathode at. a hig er direct-p0.-

tent-ialj than said first cathode, elect-rode means.

up d to said first; cath de to direct electrons therefrom in a, stream traveling; toward and impinging; upon said second cathode, electrode,v

means coupled to said second cathode, to direct. electrons therefrom in a stream traveling initially toward said first cathode but changing direction before reaching said first cathode and traveling back withthe electrons emanating from said first cathode toward said second cathode, and av cavity resonator situated along said path coupled electromagnetically to at least one of the electron streamsboth to stabilize the frequency of oscillation and to abstract oscillatory electromag: netic energy from that stream.

12. An, oscillation generator which comprises. an evacuated toroidal enclosure defining a continuous closed loop path of travel for electrons, a pair of thermionic cathodes located adjacent one another in said path, means including a sourceof direct potential to maintain, said oath; odes at. different direct potentials, electrode. means coupled to said cathodes to direct elec-- trons therefrom, and a pair of streams traveling around said path in the same direction but at discretely different velocities andv acavity reso nator situated. along said path coupled electromagnetically to at. least one of the electron streams both to stabilize the. frequency. of. oscilla tijon and to abstractoscillatory electromagnetic; energy from that stream.

13. An oscillation. generator in. accordance. with claim 12 which includes magnetic focusing. means to confine the, electrons of both streams to said path.

1,4.A high f qu n y pace discharge device. which comprises first and second thermionic. electron-emissive electrodes spacedapart to define a path of electron travel between. them, means including a source of direct potential to bias said second electrode positively with respect. to said first electrode, means coupl'ed to said, first electrod to direct a stream of. electrons from said first electrode toward and to. impinge upon said second electrode, means coupled to said second electrode. to direct a stream of electrons.

which includes secondary electronsproducedhy the impingement of the electrons from said first electrode on said second electrod toward said] ffirst electrode; and a cavity resonator "adjacent" 9 to the electron path to modulate the electron stream emanating from said second electrode under the control of the electron stream emanating from said first electrode and to withdraw oscillatory electromagnetic energy from at least one of said streams.

JOHN R. PIERCE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,157,924 Smede May 9, 1939 2,191,594 Spencer Feb. 27, 1940 Number 10 Number Name Date Spencer Feb. 27, 1940 Somers July 9, 1940 E'tzrodt May 20, 1941 Hollmann Dec. 16, 1941 Hansen et a1. Aug. 27, 1946 Smith Dec. 24, 1946 FOREIGN PATENTS Country Date Switzerland July 1, 1941 

